1. Field of the Invention
The present invention relates to an image variable magnification device and an image variable magnification method for variably magnifying an image obtained by the screen processing, by inserting or deleting pixels.
2. Description of Related Art
In an image output apparatus or the like using the electrophotographic process, the positional displacement, such as the fine displacement of the image position or the magnification between the front surface and the back surface of the recording paper, the image positional displacement between the colors (misregistration) and the like, is caused by the fixing irregularity and irregularity in the paper conveyance. In order to correct the above positional displacement, the deformation process for deforming an image in two-dimension, such as the shift, the rotation, the enlargement/reduction of an image and the like, is carried out.
As the deformation process, the enlargement/reduction process for the image is preferably realized by inserting or deleting pixels. For example, in case that an image having a dot matrix form in which the pixels are arranged in a grid form extending in two directions which are perpendicular to each other (one direction is referred to as the main scanning direction (the horizontal direction or the line direction) and the other direction is referred to as the sub-scanning direction (or the vertical direction)) is enlarged in the sub-scanning direction, in a normal nearest neighbor algorithm, pixels are inserted in each interpolation period. FIG. 18A shows an original image. FIG. 18B shows an image obtained by the enlargement processing in which the image is enlarged in the sub-scanning direction in the above nearest neighbor algorithm.
On the other hand, in the above variable magnification processing, by carrying out the processing at the higher resolution, the image quality is prevented from being deteriorated. In general, in the image processing carried out in the electrophotographic process, the screen processing is carried out for the rasterized image, and then a high-resolution image obtained by the resolution conversion is output. Therefore, as shown in FIG. 19, an image variable magnification processing unit is provided at the subsequent stage of a screen processing unit, and the variable magnification processing is carried out for the image obtained by the screen processing.
However, when the variable magnification processing is carried out for the image obtained by the screen processing, the form of the screen is deformed in the image obtained by the variable magnification processing. For example, the variable magnification processing is carried out for the image (FIG. 20A) as an input image which is obtained by carrying out the screen processing for an original image in which all of the pixels have the gray color having the same density. The input image shown in FIG. 20A is an image corresponding to the form of the screen. When the variable magnification processing is carried out for the above input image, the pixels are inserted in each interpolation period. As a result, as shown in FIG. 20B, the form of the screen is deformed.
As the technology for solving the above problem, in Japanese Patent Application Publication No. 2007-193143, the following image forming apparatus is disclosed. In the image forming apparatus, the screen pattern is deformed by shifting the dither thresholds of the screen processing unit in the direction which is reverse to the shift for carrying out the image deformation, by the amount which is the same as the amount of the above shift. As a result, after the image deformation, the screen pattern is returned to a normal form.
In case that the enlargement processing is carried out for an image, it is necessary to insert pixels in each interpolation period. As shown in FIG. 21A, the image obtained by carrying out the screen processing using the screen pattern which is previously deformed so as to cancel the shift caused by inserting pixels, is prepared. When the enlargement processing is carried out for the above prepared image as an input image, as shown in FIG. 21B, the screen pattern is returned to a normal form except the portion in which the lack of the pixel is caused due to the shift processing for enlarging the image (the above lacked pixel is referred to as “lack pixel”). However, if the gradation value of the pixel to be inserted into the position of the lack pixel is not suitably determined, the screen pattern is deformed around the lack pixel.
In Japanese Patent Application Publication No. 2007-193143, the screen pattern in the position corresponding to the lack pixel caused by the enlargement processing is extracted and stored in a memory. Then, the screen pattern stored in the memory is applied to the pixel to be inserted into the position of the lack pixel caused due to the shift processing for enlarging the input image. In this method, because it is necessary to write and read the screen pattern in/from a memory, the processing becomes complex. In particular, in case that the image processing is carried out by a hardware pipe line process, the above method is not suitable.